Nuclear sulfated oxysterol, potent regulator of cholesterol homeostasis, for therapy of hypercholesterolemia, hyperlipidemia, and atherosclerosis

ABSTRACT

The sulfated oxysterol 5-cholesten-3β, 25-diol 3-sulphate, a nuclear steroid metabolite that increases cholesterol secretion and degradation, is provided as an agent to lower intracellular and serum cholesterol and/or triglycerides. Methods which involve the use of this sulfated oxysterol to treat conditions associated with high cholesterol and/or high triglycerides (e.g. hypercholesterolemia, hyperlipidemia, and atherosclerosis) are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage CIP claiming benefits of the PCT/US2005/0338774 filed on Sep. 21, 2005, which in its turn claims priority to U.S. Provisional Application 60/621,537 filed on Oct. 25, 2004, both of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to lipid-lowering therapies. In particular, the invention provides a nuclear steroid metabolite, 5-cholesten-3β, 25-diol 3-sulphate, that increases cholesterol secretion and degradation, and is thus useful for the treatment and prevention of hypercholesterolemia, hyperlipidemia, and atherosclerosis.

2. Background of the Invention

Cholesterol is used by the body for the manufacture and repair of cell membranes, and the synthesis of steroid hormones and vitamin D, and is transformed to bile acids in the liver. There are both exogenous and endogenous sources of cholesterol. The average American consumes about 450 mg of cholesterol each day and produces an additional 500 to 1,000 mg in the liver and other tissues. Another source is the 500 to 1,000 mg of biliary cholesterol that is secreted into the intestine daily; about 50 percent is reabsorbed (enterohepatic circulation).

High serum cholesterol levels (hypercholesterolemia) are associated with the accumulation of cholesterol in arterial walls, and can result in atherosclerosis. The plaques that characterize atherosclerosis inhibit blood flow and promote clot formation, and can ultimately cause death or severe disability via heart attacks and/or stroke. A number of therapeutic agents for the treatment of hypercholesterolemia have been developed and are widely prescribed by physicians. Unfortunately, only about 35% of patients with hypercholesterolemia are responsive to the currently available therapies. Thus, there is an ongoing need to develop agents and methodologies to decrease intracellular and serum cholesterol levels.

SUMMARY OF THE INVENTION

The present invention provides a novel sulfated oxysterol, 5-cholesten-3β, 25-diol 3-sulphate, with potent cholesterol lowering properties. 5-Cholesten-3β, 25-diol 3-sulphate is a nuclear sterol metabolite that increases cholesterol secretion and degradation (bile acid synthesis). The increase in cholesterol degradation and decrease in cholesterol synthesis can lead to lower levels of intracellular and serum cholesterols. Thus, the sulfated oxysterol is useful for preventing or treating diseases associated with elevated cholesterol, such as hypercholesterolemia, hyperlipidemia, gallstone, cholestatic liver disease, and atherosclerosis.

It is an object of this invention to provide a substantially purified 5-cholesten-3β, 25-diol 3-sulphate having the following chemical formula:

It is a further object of the invention to provide a cholesterol-lowering composition.

The composition comprises 5-cholesten-3β, 25-diol 3-sulphate, and a pharmaceutically acceptable carrier.

It is a further object of the invention to provide a method for lowering serum cholesterol and triglyceride levels in a patient in need thereof. The method comprises the step of administering 5-cholesten-3β, 25-diol 3-sulphate to the patient in an amount sufficient to lower serum cholesterol and triglyceride levels in the patient.

The invention further provides a method to treat or prevent pathological conditions associated with high serum cholesterol and triglyceride levels in a patient in need thereof. The method comprises the step of administering 5-cholesten-3β, 25-diol 3-sulphate to the patient in an amount sufficient to lower serum cholesterol levels in the patient, and to prevent or treat the pathological condition. The pathological condition is, for example, hypercholesterolemia, hyperlipidemia, or atherosclerosis.

The invention further provides a method of increasing cholesterol secretion or degradation in cells. The method comprises the step of increasing a level of 5-cholesten-3β, 25-diol 3-sulphate in the cells. The method may be carried out by exposing the cells to 5-cholesten-3β, 25-diol 3-sulphate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Subcellular fractionation protocol. For details see “Experimental Procedures”.

FIG. 2A-C. Effects of overexpression of StarD1 on Cyp7A1 mRNA expression in primary rat hepatocytes. At day five following infection of virus control, virus encoding StarD1, and CYP27A1 as indicated, cells were harvested and total RNAs were extracted. To each lane, 20 μg of total RNA was loaded. Specific Cyp7A1 mRNAs (Panel A) were determined by Northern blot analysis. Cyclophilin was used as control (Panel B). The expressions of Cyp7A1 mRNAs were increased by 6-fold (n=3) following overexpression of StarD1 and 2.5 fold following CYP27A1 as indicated (Panel C).

FIG. 3. Thin layer chromatographic analysis of the chloroform extractable cholesterol derivatives. Rat primary hepatocytes were infected with the indicated viruses. Forty-eight hrs later cells were harvested and nuclear lipids extracted and analyzed as explained under “Experimental Procedures”.

FIGS. 4A and B. Phase distribution of [¹⁴C]cholesterol derivatives in nuclei of primary rat hepatocytes following overexpression of StarD1 and CYP7A1. Rat primary hepatocytes were infected with the indicated viruses. Forty-eight hrs later cells were harvested and subcellular fractions prepared. Fractions E and F were processed for lipid analysis as explained under Experimental Procedures”. A, Nuclear inner membrane (Fraction E). B, Nuclear digests (Fraction F).

FIG. 5A-C. HPLC analysis of [¹⁴C]cholesterol derivatives in the nuclear fraction (Fraction D) and non-nuclear fraction (Fraction A). Twenty-four hrs following the indicated recombinant adenovirus infection, cells were harvested and nuclear and non-nuclear fractions (Fractions A and D) were isolated, extracted by the Folch method and the methanol/water phase analyzed. A, nuclear extracts (Fraction D) 195 nm profiles. B, nuclear extracts (Fraction D) ¹⁴C profiles. C, non-nuclear extracts (Fraction A) ¹⁴C profiles. In each case, nuclear methanol/water extracts of the equivalent of 5×10⁶ cells were loaded.

FIG. 6A-F. HPLC analysis of the cholesterol derivatives extracted from the nuclei, mitochondria, and culture media. Rat primary hepatocytes were infected with the StarD1 adenovirus and two hrs later [¹⁴C]cholesterol was added to the media. Twenty-four hrs following, cells and culture media were harvested. Nuclei and mitochondria were isolated as described under “Experimental Procedures”. Total lipids were extracted from the nuclei, mitochondria, and culture media by Folch partitioning into methanol phase, and analyzed by HPLC as described in “Experimental Procedures”. A-C, 195 nm profiles. D-F, radioactivity profiles.

FIG. 7A-D. Characterization of the nuclear oxysterol by enzymatic digestion followed by HPLC and TLC. Nuclear [¹⁴C]oxysterol derivatives were isolated from StarD1 overexpressing rat primary hepatocytes and digested with 1 mg/ml of sulfatase in acetic acid buffer, pH 5.0, overnight. Total lipids were extracted with chloroform/methanol and separated by Folch partitioning. The products in the chloroform and methanol/water phase were analyzed by HPLC using a mixture of 965 ml hexane, 25 ml isopropanol, and 10 ml acetic acid as mobile phase, 1.3 ml/min flow rate and ¹⁴C quantified. A, HPLC elution profile of the sulfatase digestion products. B, HPLC elution profile of [¹⁴C]27-hydroxycholesterol. C, HPLC elution profile of [¹⁴C]25-hydroxycholesterol. D, products from the chloroform phase were further analyzed by TLC using a mixture of toluene:ethyl acetate (2:3) as developing solvent. 27-C represents 27-hydroxycholesterol; P, sulfatase digestion products of the purified nuclear oxysterols; 25-C, 25-hydroxycholesterol.

FIG. 8A-C. Characterization of nuclear oxysterol by negative ion-triple quadruple mass spectrometry (LC/MS/MS). (A) A selected ion chromatogram of mass ion at m/z 481; (B) the Q1 full scan spectrum; (C) product scan spectrum of a/z 481. The amu represents atomic mass units, and cps, counts per second.

FIG. 9A-B. Effect of the nuclear oxysterol on cholesterol uptake and bile acid biosynthesis. Rat primary hepatocyes were treated with nuclear extracts (methanol/water phase) (A and B) or purified nuclear oxysterol dissolved in control nuclear extract (C) 24 hrs after plating them. Then [¹⁴C]cholesterol was added as described in FIG. 1. Culture media were then harvested at 0, 6, 12, and 24 hrs and radioactivity quantified (A). Bile acid synthesis rates (B and C) were measured as the conversion of [¹⁴C]cholesterol into methanol/water extractable counts as described in (9).

FIG. 10. ABCA1 (A1), ABCG1 (G1), LDL receptor (LDLR), ABCG5 (G5), and ABCG8 (G8) gene expression in primary mouse hepatocytes following addition of the purified nuclear oxysterol. At 24 hrs following the addition, cells were harvested and total RNAs were extracted and gene expression levels were quantitated by real time RT-PCR. β-Actin mRNA was used as total mRNA internal standard. The gene expression levels in cells with StarD1 overexpression were compared with those in control cells. Ten μg of total RNA was used for cDNA preparation (RT) and 10 ng of cDNA was used for PCR. The expression levels were normalized to β-actin.

FIG. 11. Biosynthesis pathway of nuclear sulfated oxysterol. In the presence of StarD1 protein, cholesterol is delivered into mitochondria where 25-hydroxylase (25-OHLase) and hydroxylcholesterol sulfate transferase 2b (HST2b) locate, and converted to be 25-hydroxycholesterol 3-sulfate. This sulfated oxysterol translocates to nucleus and regulates gene expressions involved in cholesterol metabolism.

FIG. 12A-D. (A) addition of sulfate group onto 3β-position of 25-hydroxycholesterol for the synthesis of the novel nuclear oxysterol by incubation with sulfur trioxide triethyl amine complex; (B) mass spectrophometric analysis of the product after incubation with the sulfur trioxide and purified by HPLC. Mass ion, m/z 481, represents 25-hydroxycholesterol (M.W. 482)+Sulfate group (M.W.80); (C) nuclear magnetic resonance (NMR) analysis of the 25-hydroxycholesterol 3-sulfate as starting material for the synthesis. The chemical shift of the proton at C3 in the molecule can be seen at 3.35 ppm; and (D) NMR analysis of the product shows the proton at C3 in the molecule has been shifted to 4.12 ppm from 3.35 ppm in its original compound.

FIG. 13A-C. TLC and HPLC analysis of the newly synthesized [¹⁴C]cholesterol. After incubation of the 25HC3S-treated cells with [1-¹⁴C]acetate for 2 hrs, the cells were harvested. The total neutral lipids were extracted with chloroform/methanol and partitioned into chloroform phase. The [¹⁴C]-acetate derivatives were analyzed by thin layer chromatography (TLC) and HPLC. A. TLC analysis of the [¹⁴C]-acetate derivatives: the chloroform phase extracts of the equivalent of 5×10⁶ cells were loaded onto each lane, separated by developing system of tuluene:acetyl acetate, and visualized by Imagine Reader. B. HPLC analysis of the [¹⁴C]-acetate derivatives: the chloroform phase extracts of the equivalent of 5×10⁶ cells were loaded onto silica gel column. The effluents were collected, 0.5 min/fraction. The radioactivities in each fraction were determined by Scintillation Counting. C. A summary of three experiments of HPLC analysis. Each bar represents the mean of three experiments±standard deviation.

FIG. 14A-H. HPLC analysis of cholesterol levels in microsomal fractions. The total lipids were extracted from 25HC3S-treated (left panels) or 25HC- (right panels) treated HepG2 cells. α,β-Unsaturated ketones were generated by incubating the extracted sterols with cholesterol oxidase and were analyzed by normal phase HPLC: Panels A-C: the lipids from the cells treated with 0, 3, and 6 μM of 25HC3S; D. A summary of a series experiments, the lipids from the cells treated with 0, 3, 6, 12, and 25 μM of 25HC3S. Panels E-G: the lipids from the cells treated with 0, 3, and 6 μM of 25HC; D. A summary of a series experiments, the lipids from the cells treated with 0, 3, 6, 12, and 25 μM of 25HC. The data represent a typical result from one of three independent experiments.

FIG. 15A-D. 25HC3S regulates HMGR mRNA Expression. Real time RT-PCR analysis of HMG CoA R reductase expression in HepG2 cells (Panels A and B). Western blot analysis of protein levels of HMG CoA reductase (Panel C). Total RNAs were purified using SV Total RNA Isolation Kit (Promega) from the cells treated with 25HC3S at concentrations as indicated (A) and incubation at 12 μM of 25HC3S for the different times (effect on HMG CoA mRNA) (Panel B). Two μg of total RNA was used for cDNA preparation (RT) and performed as manufacture recommended (Invitrogen), and 10 ng of cDNA was used for real time PCR. The expression levels were normalized to GAPDH.

The total extracted protein, 100 μg, was loaded in each well for each condition as indicated in panel C. The bands of HMG CoA reductase (HMGR) were qualitated by laser density scanning. The data from three experiments were summarized in Panel D. Each value represents mean of three experiments±standard derivation.

FIG. 16A-C. Effects of 25HC and 25HC3S on the levels of HMG CoA reductase mRNA in hepatocytes. Total RNA was extracted from the HepG2 cells cultured either in 10% FBS media (Panel A) or in 10% lipid-depleted sera (Panel B) and treated with either 25HC or 25HC3S as indicated. The real time RT-PCR analysis was performed as described in FIG. 16. The data represent a typical result from one of three independent experiments. The 25HC or 25HC3S affects the HMG CoA reductase expression in primary human hepatocytes (Panel C). The data represent a typical result from one of three independent experiments.

FIG. 17A-E. HPLC analysis of 25HC in PHH cells treated with 25HC. The total lipids were extracted from 25HC-treated PHH cells. α,β-Unsaturated ketones were generated by incubating the extracted sterols with cholesterol oxidase and were analyzed by normal phase HPLC: Panels A-E: the lipids from the cells treated with 0, 3, 6, 12, 25 μM of 25HC as indicated; The data represent a typical result from two independent experiments.

FIG. 18A-D. Western blot analysis of SREBPs activation following 25HC3S or 25HC treatment in HepG2 cells. Total proteins were extracted from HepG2 cells treated with 25HC in ethanol (0.1%) (Panel A and C) or 25HC3S in DMSO (0.1%) (Panel B and D), cultured in the media in absence (Panels A and B) or presence (Panels C and D) of mevinolin (50 mM) and mevalonate (0.5 mM). SREBP-1 and SREBP-2 protein levels in the cells were determined by Western blot analysis. The extracted protein (100 μg) was loaded onto each lane for each condition as indicated. The data represent a typical result from one of three independent experiments. The trends of the protein levels are highly reproducible.

FIG. 19. Relative levels of cholesterol in HepG2 microsomal fractions (y axis) vs concentration of 25HC3S (x axis).

FIG. 20. Lipids in mouse sera after injection of 25HC3S.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is based on the discovery of 5-cholesten-3β, 25-diol 3-sulphate, a novel sulfated oxysterol with potent cholesterol lowering properties. The chemical structure of the sulfated oxysterol is as follows:

This sulfated oxysterol is a nuclear sterol metabolite that decreases cholesterol and triglyceride levels in serum. The compound increases cholesterol secretion by increasing expression of cholesterol transporters in hepatocytes. The increase in cholesterol secretion and degradation ultimately leads to lower levels of serum cholesterol. Without being bound by theory, it appears that the sulfated oxysterol is made in the mitochondrion and translocates to the nucleus of the cell, where it acts to up-regulate genes involved in cholesterol metabolism. 5-Cholesten-3β, 25-diol 3-sulphate is thus useful for preventing or treating diseases associated with elevated cholesterol (hypercholesterolemia), such as hyperlipidemia, atherosclerosis, coronary heart disease, stroke, and cholestatic liver disease. This sulfated oxysterol is especially suitable for in vivo use because it is a biosynthesized authentic compound (hydroxylation and sulfation of cholesterol) in vivo. Thus, 5-cholesten-3β, 25-diol 3-sulphate should have few or no toxic side effects when administered to patients. In addition, the invention provides methods of preparing and administering 5-cholesten-3β, 25-diol 3-sulphate.

The compound of the invention will be provided in a substantially purified form for use in the methods of the invention. By “substantially purified” we mean that the sulfated oxysterol is provided in a form that is at least about 75%, preferably at least about 80%, more preferably at least about 90%, and most preferably at least about 95% or more free from other chemical species, e.g. other macromolecules such as proteins or peptides, nucleic acids, lipids, and other cholesterol-related species (e.g. other cholesterol derivatives such as cholesterol metabolites, chemically modified forms of cholesterol such as various other hydroxylated cholesterol species, etc.). In one embodiment of the invention, the sulfated oxysterol of the invention may be isolated and purified from living cells. One embodiment of this method is described in Example 1 in the Examples section below. However, those of skill in the art will recognize that in order to generate larger quantities of the sulfated oxysterol, the compound may also be synthesized, either by synthetic chemical means, or by methods which involve the use of recombinant DNA technology (e.g. by using cloned enzymes to carry out suitable modifications of cholesterol). An exemplary synthesis scheme for 5-cholesten-3β, 25-diol 3-sulphate is as follows: A mixture of 25-hydroxycholesterol (0.1 mmol) and sulfur trioxide triethyl amine complex (0.12 mmol) in dry toluene was heated to 60 degree for 24 hours under nitrogen, then cooled and the solvent was evaporated under reduced pressure. The residue was purified by flash chromatography to afford the product as a white solid.

The methods of the invention are useful for the treatment or prevention of conditions associated with high levels of cholesterol and triglyceride (hyperlipidemia). Such conditions may be either caused or exacerbated by high cholesterol and triglyceride, and include but are not limited to hyperlipidemia, atherosclerosis, heart disease, stroke, Alzheimer's, gallstone diseases, cholestatic liver diseases, etc. By “treat” we mean that a disease condition has already developed, and the methods of the invention are used to ameliorate symptoms of the disease condition, either to stop or decrease progression of the disease, or to reverse symptoms of the disease, either partially or fully. Alternatively, by “prevent” we mean that the compounds of the present invention may be administered to patients prophylactically prior to the development of disease symptoms, e.g. to one who has high cholesterol but has not yet developed atherosclerosis, or to one who does not yet have high cholesterol but is at high risk for developing high cholesterol (e.g. as determined by genetic factors, family history, etc.)

Those of skill in the art will recognize that the phrase “high cholesterol” generally relates to cholesterol levels in serum in the range of about 200 mg/dl or more. A determination of “high cholesterol” is typically made by a health professional such as a physician, and the established meaning of “high cholesterol” may vary somewhat from professional to professional. Further, the precise definition may vary somewhat depending on the state of the art, e.g. on findings from studies which investigate the relationship between cholesterol levels and diseases. Nevertheless, those of skill in the art will be able to identify suitable candidates for administration of the sulfated oxysterol of the present invention. By “lowering cholesterol levels” we mean that the level of free serum cholesterol in a patient is decreased by at least about 10% to 30%, and preferably at least about 30 to 50%, and more preferably at least about 50 to 70%, and most preferably at least about 70 to about 100%, or more, in comparison to the level of cholesterol in the patient prior to administration of the sulfated oxysterol. Alternatively, the extent of the decrease may be determined by comparison to a similar untreated control individual to whom the compound is not administered. Those of skill in the art are familiar with such determinations, e.g. the use of controls, or the measurement of cholesterol levels in the blood before and after administration of an agent that lowers cholesterol.

Implementation of the methods of the invention will generally involve identifying patients suffering from or at risk for developing conditions associated with high cholesterol, and administering the compound of the present invention in an acceptable form by an appropriate route. The exact dosage to be administered may vary depending on the age, gender, weight and overall health status of the individual patient, as well as the precise etiology of the disease. However, in general for administration in mammals (e.g. humans), dosages in the range of from about 0.1 to about 100 μg or more of compound per kg of body weight per 24 hr., and preferably about 0.1 to about 50 μg of compound per kg of body weight per 24 hr., and more preferably about 0.1 to about 10 μg of compound per kg of body weight per 24 hr. are effective.

Administration may be oral or parenteral, including intravenously, intramuscularly, subcutaneously, intradermal injection, intraperitoneal injection, etc., or by other routes (e.g. transdermal, sublingual, oral, rectal and buccal delivery, inhalation of an aerosol, etc.). In a preferred embodiment, administration is oral. Further, administration of the compound may be carried out as a single mode of therapy, or in conjunction with other therapies, e.g. other cholesterol lowering drugs, exercise and diet regimens, etc.

The compounds may be administered in the pure form or in a pharmaceutically acceptable formulation including suitable elixirs, binders, and the like (generally referred to a “carriers”) or as pharmaceutically acceptable salts (e.g. alkali metal salts such as sodium, potassium, calcium or lithium salts, ammonium, etc.) or other complexes. It should be understood that the pharmaceutically acceptable formulations include liquid and solid materials conventionally utilized to prepare both injectable dosage forms and solid dosage forms such as tablets and capsules and aerosolized dosage forms. In addition, the compounds may be formulated with aqueous or oil based vehicles. Water may be used as the carrier for the preparation of compositions (e.g. injectable compositions), which may also include conventional buffers and agents to render the composition isotonic. Other potential additives and other materials (preferably those which are generally regarded as safe [GRAS]) include: colorants; flavorings; surfactants (TWEEN, oleic acid, etc.); solvents, stabilizers, elixirs, and binders or encapsulants (lactose, liposomes, etc). Solid diluents and excipients include lactose, starch, conventional disintegrating agents, coatings and the like. Preservatives such as methyl paraben or benzalkium chloride may also be used. Depending on the formulation, it is expected that the active composition will consist of about 1% to about 99% of the composition and the vehicular “carrier” will constitute about 1% to about 99% of the composition. The pharmaceutical compositions of the present invention may include any suitable pharmaceutically acceptable additives or adjuncts to the extent that they do not hinder or interfere with the therapeutic effect of the sulfated oxysterol.

The administration of the compound of the present invention may be intermittent, or at a gradual or continuous, constant or controlled rate to a patient. In addition, the time of day and the number of times per day that the pharmaceutical formulation is administered may vary and are best determined by a skilled practitioner such as a physician. Further, the effective dose can vary depending upon factors such as the mode of delivery, gender, age, and other conditions of the patient, as well as the extent or progression of the disease condition being treated. The compounds may be provided alone, or in combination with other medications or treatment modalities.

In addition, the compound of the invention may also be used for research purposes.

EXAMPLES Example 1

Sterol ligands play key roles in maintenance of the cholesterol homeostasis. The present study has identified a novel regulatory nuclear sulfated oxysterol, which is generated in mitochondria, translocates to the nucleus, and upregulates the rate of bile acid synthesis. At forty-eight hrs after infection with recombinant adenovirus encoding a mitochondria cholesterol transport protein (StarD1) in primary rat hepatocytes, bile acid synthesis increased by 5-fold. Concurrently, [¹⁴C]oxysterol derivatives with retention time at 11.50 min in HPLC elution profile was dramatically increased both in the mitochondria and in the nucleus, but not in culture media. The oxysterol product could be extracted into the chloroform phase from the methanol/water phase after sulfatase treatment, and had the same physical properties as 25-hydroxycholesterol. LC/MS/MS analysis showed the nuclear oxysterol with a molecular ion, m/z 481, in the Q1 full scan spectrum, and the presence of fragment ions at m/z 59, 80, 97, and 123 in its product scan spectrum. Thus, the nuclear oxysterol derivative can be characterized as 5-cholesten-3β, 25-diol 3-sulphate. The addition of nuclear extract from the cells overexpressing StarD1 or the addition of the purified oxysterol to primary rat hepatocytes significantly increased the rates of bile acid synthesis (>3.5 fold), suggesting this oxysterol derivative is an active regulator. These results provide evidence for a new regulatory pathway by which a novel potent regulatory nuclear sulfated oxysterol is generated in mitochondria, translocates to the nucleus, and upregulates bile acid synthesis.

Introduction

The biotransformation of cholesterol to primary bile acids occurs via two main pathways in hepatocytes (1). The “neutral” pathway is considered to be the major pathway at least in humans and rats (2). In this pathway the sterol nucleus is modified before the side-chain, beginning with hydroxylation of cholesterol at the 7α position. This reaction is catalyzed by cholesterol 7α-hydroxylase (CYP7A1), the first and rate-limiting step of this pathway. The ability to lower plasma cholesterol levels via the pharmacological control of CYP7A1 expression represents a therapeutic approach that has been in use for the last 30 years and is still of intense research interest. Multiple negative and positive modulators of CYP7A1 transcription have been identified both in tissue culture systems and in vivo (3) and many of these modulators are oxysterols, such as hydroxy-cholesterol molecules and bile acids. They function by activating nuclear receptors, such as liver X receptor (LXR) and farnesoid X receptor (FXR), which in turn regulate the expression of regulatory genes involved in bile acid biosynthesis, such as CYP7A1 and sterol 12α-hydroxylase (CYP8B1), the enzyme specific for cholic acid synthesis. Oxysterols are also key regulatory molecules for the expression of many other genes involved in the homeostasis of cholesterol, and other lipids, such as 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase, low density lipoprotein (LDL) receptor, some ATP-binding cassette transporters, like the ABCA1 and ABCG8, and many others. They function by modulating the activity of either nuclear receptors or other transcriptional factors, such as the sterol regulatory binding proteins (SREBPs) (4-6). Thus, characterizing endogenous synthesized oxysterols and their mechanism of action is critical for a better understanding of lipid homeostasis. The initial step in the “acidic” pathway is catalyzed by the enzyme mitochondrial sterol 27-hydroxylase (CYP27A1). The oxysterol intermediates of the “acidic” pathway such as 25-, or 27-hydroxycholesterol have been shown in vitro to be potent regulators in cholesterol homeostasis (7). Increased CYP27A1 activity in peripheral tissues may both down-regulate cholesterol synthesis through the SREBP pathway, and enhance the efflux of cholesterol and its elimination via LXR (8). However, the physiological and authentic in vivo LXR ligand is unknown (9).

We have recently found that overexpression of steroidogenic acute regulatory protein (StarD1), a protein which facilitates cholesterol transport to the mitochondria, dramatically increases cholesterol transport into mitochondria, the hydroxylation of cholesterol to oxysterol, and cholesterol catabolism to bile acids both in primary hepatocytes in culture and in vivo (10;11). This suggests that cholesterol delivery to the mitochondria, where the enzyme CYP27A1 is localized, is the rate-determining step for bile acid synthesis via the acidic pathway. Subsequently, StarD1 was found in isolated hepatocytes (12). Overexpression of StarD1 in vivo increases bile acid synthesis not only to the same level as overexpression of CYP7A1, but also produces a similar composition of bile acids in mouse bile in vivo (11). Thus, it is reasonable to hypothesize that potent oxysterol molecule(s) might be generated in the mitochondria, thereby regulating bile acid synthesis, and playing an important role in maintenance of intracellular cholesterol homeostasis.

To test this hypothesis, a recombinant adenovirus encoding StarD1 was used to overexpress StarD1 in primary rat hepatocytes in order to increase bile acid synthesis. In this study we present evidence for previously unappreciated sulfated oxysterols in the nucleus of cells infected with the StarD1 adenovirus. The chemical structure of the most abundant nuclear oxysterol was characterized by HPLC, Triple Quadrupole LC/MS/MS, enzymatic digestion, and TLC analysis and was identified as 5-cholesten-3β, 25-diol 3-sulphate. We also provide evidence for a potential function(s) of this newly identified nuclear oxysterol in cholesterol catabolism.

Abbreviations:

The abbreviations used are: CYP8B1, sterol 12α-hydroxylase; CYP27A1, cholesterol 27-hydroxylase; CMV, cytomegalovirus; CYP7A1, cytochrome P450 7α-hydroxylase; FXR, farnesil X receptor; SRE, sterol regulatory element; SREBP, sterol regulatory binding protein; HMG-CoA, 3-hydroxy-3-methylglutaryl-coenzyme A; Q-RT-PCR, quantitative reverse transcription PCR.

Experimental Procedures

Materials

Cell culture reagents and supplies were purchased from GIBCO BRL (Grand Island, N.Y.). [¹⁴C]Cholesterol and [³H]25-Hydroxycholesterol were purchased from New England Nuclear (Boston, Mass.). [¹⁴C]27-Hydroxycholesterol was prepared as previously described (13). Cyclodextrin was purchased from Cyclodextrin Technologies Development Inc. (Gainsville, Fla.). Silica gel thin-layer chromatography plates (LK6 D) were from Whatman (Clifton, N.J.). Silica gel 1B TLC sheets were purchased from VWR (Bridgeport, N.J.). All other reagents were from Sigma Chemical Co (St. Louis, Mo.), unless otherwise indicated.

Adenovirus Preparation and Propagation

The adenovirus construct used in this study was obtained through the Massey Cancer Center Shared Resource Facility of the Virginia Commonwealth University as previously described (14).

RNA Preparation and Quantification

RNA was isolated and CYP7A1 was quantified using Northern blot assays (20 μg of total RNA) as previously described (15).

Culture and Subcellular Fractionation of Primary Rat Hepatocytes and Lipid Fractionation

Primary rat hepatocyte cultures, prepared as previously described (16), were plated on 150 mm tissue culture dishes (˜2.5×10⁷ cells) in Williams' E medium containing dexamethasone (0.1 μM). Cells were maintained in the absence of thyroid hormone. Twenty-four hrs after plating, culture medium was removed, and 2.5 ml of fresh medium was added. Cells were then infected with recombinant adenovirus encoding either the StarD1 (Ad-CMV-StarD1) or the CYP7A1 (Ad-CMV-CYP7A1) cDNAs in front of the human cytomegalovirus promoter (CMV) or no cDNA, as a control virus. The viruses were allowed to incubate for at least 2 hrs in minimal culture medium with gentle shaking of the plates every 15 min. After 2 hrs of infection, unbound virus was removed, replaced with 20 ml of fresh medium, and 2.5 μCi of [¹⁴C]cholesterol was added. After 48 hrs, cells were then harvested and processed for nuclei isolation as described (17) with minor modification (FIG. 1). Briefly, cells were disrupted by Dounce homogenization in buffer A (10 mM HEPES-KOH at pH 7.6, 1.5 mM MgCl₂, 10 mM KCl, 0.5 mM dithiothreitol, 1 mM sodium EDTA, 1 mM EGTA) and spun at 1,000×g for 10 min. The nuclear pellet was further fractionated by resuspension in 2.5 ml of a 1:1 mixture of buffer A and buffer B (2.4 M Sucrose, 15 mM KCl, 2 mM sodium EDTA, 0.15 mM spermine, 0.15 mM spermidine, 0.5 mM dithiothreitol) and centrifuged at 100,000×g for 1 hr at 4° C. through a 1 ml cushion of 3:7 mixture of buffer A and B. The washed nuclear pellet was resuspended in buffer A containing 0.5% (v/v) Nonidet P-40 and centrifuged at 1000×g for 10 min at 4° C. The supernatant is designated as nuclear attached membrane (fraction C) and the pellets as purified nuclei (fraction D).

The purified nuclei (Fraction D) were resuspended and digested by 2 mg/ml of DNase I in 50 mM of acetic buffer, pH 5.0, 10 mM MgCl₂ at 37° C. for 2 hrs. After centrifugation at 10,000×g for 20 min, the supernatant was designed as inner nuclear membrane (fraction E). The pellets were further digested by 2 mg/ml of proteinase K in phosphate buffered saline solution (PBS) at 50° C. for 16 hrs and the solution was designed as nuclear protease digests (fraction F). Total lipids in each fraction were extracted by adding 3.3 volumes of chloroform:methanol (1:1) and separated into two phases, methanol/water and chloroform phases as previously described (18). The counts of [¹⁴C]cholesterol derivatives in the methanol/water and chloroform phases were measured by liquid scintillation counting (LC 60001C, Beckman, Fullerton, Calif.).

TLC and HPLC Analysis of Cholesterol Derivatives

The [¹⁴C]cholesterol/cholesterol derivatives in chloroform and methanol/water phases were examined by thin layer chromatograph (TLC) (E. Merck, Darmstadt, Germany) using different developing solvent systems: toluene:ethyl acetate (2:3, v/v) for the [¹⁴C]cholesterol/oxysterols in chloroform phase, and ethyl acetate:cyclohexane:acetic acid (92:28:12; v/v/v) for those in methanol/water phases. The [¹⁴C]cholesterol/cholesterol derivatives were visualized in Phosphorimager using Fuji imaging plates (Fujifilm BAS-1800II, Fuji Photo Film Co., LTD, Japan).

Total [¹⁴C]-cholesterol derivatives in chloroform phase were analyzed by HPLC on an Ultrasphere Silica column (5μ×4.6 mm×25 cm; Backman, USA) using HP Series 1100 solvent delivery system (Hewlett Packard, Japan) at 1.3 ml/min flow rate. The column was equilibrated and run in a solvent, Hexane:Isopropanol:Glacial Acetic Acid (965:25:10, v/v/v), as the mobile phase. The effluents were collected every 0.5 min (0.4 ml per fraction) except as indicated. The counts of [¹⁴C]cholesterol/cholesterol derivatives were determined by Scintillation Counter. The column was calibrated with cholesterol, [³H]25-hydroxycholesterol, and [¹⁴C]27-hydroxycholesterol.

Total [¹⁴C]-cholesterol derivatives found in methanol/water phases were analyzed on an Ultrasphere PTH C-18 column (5μ×4.6 mm×25 cm; Backman, USA) at 0.8 ml/min flow rate. The column was equilibrated and run in 20 mM KH₂PO₄, pH 4.2:acetonitrile:methanol (1:3:6, v/v/v) as the mobile phase. The effluents were monitored at 195 nm and collected every 0.5 min (0.4 ml per fraction) except as indicated. The column was calibrated with tauroursodeoxycholic acid, glycoursodeoxycholic acid, taurocholic acid, glycocholic acid, taurochenodeoxycholic acid, taurodeoxycholic acid, and progesterone.

Sulfatase Treatment of Purified Nuclear [¹⁴C]Cholesterol Derivatives:

The purified nuclear [¹⁴C]cholesterol derivatives were digested with 2 mg/ml of sulfatase (EC 3.1.6.1) (Sigma, St Louis, Mo.) in 50 mM of acetic buffer, pH 5.0 by incubation at 37° C. for 4 hrs. The products were extracted into chloroform phase from methanol/water phase by adding 3.3 volume of methanol:chloroform (1:1, v/v) to reaction solution. [¹⁴C]Cholesterol derivatives in both chloroform and methanol/water phases were then analyzed by TLC and HPLC as stated above.

Reverse Phase Liquid Chromatography/Mass Spectrometry/Mass Spectrometry (LC/MS/MS) Analysis of Nuclear Cholesterol Derivatives:

Reverse phase liquid chromatographic separation was performed on an HP series 1100 system (Agilent Technologies, Palo Alto, Calif.) and CTC HTS-PAL autosampler (Leap Technologies, Carrboro, N.C.). Separation was carried on a ThermoKeystone Aquasil C18 column (5 μm, 2.1 mm×100 mm). The mobile phase consisted of (A), 0.1% formic acid in water, and (B), 0.1% formic acid in acetonitrile. The 20 min gradient was as follows: 0-10.0 min, 10%-95% B linear; 10.0-15.0 min, 95% B; 15.0-15.1 min, 95%-10% B linear; 15.1-20.0 min, 10% B. The mass detector was an API 4000 (MDS Sciex, Toronto, Canada)

The elution stream (0.3 ml/min) from the HPLC apparatus was introduced into a MDS Sciex API 4000 Triple Quadrapole Mass Spectrometer with a Turbo IonSpray ionization (ESI) source for the analyses. The mass spectrometer was operated in negative ion modes and data were acquired using both full scan mode as well as the product ion mode for MS/MS.

The sample was reconstituted into methanol:water (20:80, v/v). The solution containing the fraction of 11.50 min peak was infused into the LC/MS/MS system to optimize ESI-MS-MS parameters. The optimized parameters for Q1 full scan under the negative mode were: CUR: 10; GS1: 40; GS2: 40; TEM:400; IS: −4500; DP: −150; EP:−10. The optimized parameters for the product scan of 481 under the negative mode were: CUR:10; GS1: 40; GS2: 40; TEM: 400; IS: −4500; CAD: 5; DP: −150; EP: −10; CE: 50; CXP: −15.

Bile Acid Biosynthesis and Analysis:

Bile acid synthetic rates were determined by the addition of 2.5 μCi of [¹⁴C]cholesterol to each P150 mm plate of confluent primary rat hepatocyte cultures (˜2.5×10⁷ cells) 24 hrs after plating. Media and cells were harvested 48 hrs after viral infection. Conversion of [¹⁴C]cholesterol into [¹⁴C]-methanol-water soluble products was determined by scintillation counting after extraction with chloroform-methanol (2:1, vol/vol) of cells and of culture media. Rates of bile acid biosynthesis following recombinant adenovirus infection were calculated as the ratio of [¹⁴C]-methanol-water soluble counts to the sum of chloroform plus methanol-water counts. Individual bile acids were identified by HPLC analysis as described above.

Time Course of Bile Acid Synthesis

Time points for conversion of [¹⁴C]-cholesterol to [¹⁴C]-bile acids were carried out using P150 mm tissue culture dishes Aliquots (1/100) of media were collected in duplicate in a microfuge tube and kept frozen until analysis. A mini Folch extraction was carried out by adding 3 volume of methanol:chloroform (1:1) to the culture medium. The tubes were vigorously vortexed and centrifuged at 16,000 g for 6 min. The phases were collected separately and counted.

Statistics:

Data are reported as mean ±standard error. Where indicated, data were subjected to t-test analysis and determined to be significantly different if P<0.05.

Results

A Novel Nuclear Oxysterol is Generated in Mitochondria and Translocated to the Nucleus in Primary Rat Hepatocytes upon StarD1 Overexpression.

Primary rat hepatocytes were infected with an adenovirus encoding either StarD1 or CYP7A1 as explained in Experimental Procedures. Forty-eight hrs after infection, bile acid synthesis increased 4-fold and 7-fold respectively, as we previously reported (18). Cells infected with control virus have similar levels of bile acid synthesis as uninfected cells. To test whether overexpression of StarD1 affects the expression of LXR targeting genes following generation of the nuclear oxysterol, the regulation of CYP7A1 expression was investigated following StarD1 overexpression. CYP7A1 mRNA levels increased by 6-fold (n=3) at day 3 following StarD1 overexpression (FIG. 2).

The subcellular distribution of [¹⁴C]-cholesterol derivatives was monitored by adding exogenous [¹⁴C]cholesterol to the hepatocyte culture 2 hrs after infection and is summarized in Table 1. Approximately 50% of the total counts of [¹⁴C]cholesterol/cholesterol derivatives were found in nuclear-related fractions. The other 50% were located in other subcellular organelles including cytosol, plasma membranes, lysosomes, and mitochondria (Fractions A and B). Only a small number of counts were detected in the inner membrane fraction (Fraction E). Interestingly, the total extractable [¹⁴C]cholesterol/cholesterol derivatives in nuclear debris from StarD1-infected cells was significantly (25%) higher than those of CYP7A1-infected or control nuclear extracts (Fraction F). TLC analysis of the chloroform-extractable cholesterol derivatives found in the nuclear fraction (Fraction D) in StarD1-infected hepatocytes showed four bands, which migrated like cholesterol ester, cholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol respectively (FIG. 3). Only cholesterol ester, cholesterol, and 27-OH cholesterol were detected in control-infected cells, with cholesterol ester and cholesterol in CYP7A 1-infected cells. These results suggested that StarD1 overexpression increased translocation of cholesterol derivatives, 25-hydroxycholesterol, to the nucleus. The presence of 25-hydroxycholesterol in the nucleus following StarD1 overexpression has been confirmed by mass spectrometry/mass spectrometry (MS/MS) analysis (data not shown). 25-Hydroxycholesterol is a minor oxysterol that may be formed in different types of tissues by a specific enzyme that may not belong to the cytochrome P-450 family (19). Interestingly, cholesterol-derivatives in the methanol/water phase extracted from the nuclear debris were dramatically increased in the StarD1-overexpressing cells compared with control and CYP7A1-overexpressing cells (FIG. 4). TABLE 1 Distribution of [¹⁴C]cholesterol derivatives in primary rat hepatocytes following StarD1 or CYP7A1 overexpression*. Subcellular Non-nuclear Nuclear Fraction A B C E F Control 43 ± 2  23 ± 5 47 ± 3 1.0 ± 0.2 7 ± 3 StarD1 38 ± 8 3.1 ± 2 36 ± 6 1.7 ± 0.1 10 ± 2  CYP7A1 43 ± 3 2.9 ± 4 49 ± 8 1.1 ± 0.2 8 ± 2 *[¹⁴C]Cholesterol was added at 2 hrs after adenovirus infection and cells were harvested 48 hrs later. Subcellular fractions were prepared as described under “Experimental Procedures”. An aliquot from each fraction was taken for liquid scintillation counting. Values represent the mean of three experiments ± SD of the percentage of radioactivity found in each fraction with respect to the total radioactivity found in all fractions.

We then proceeded to analyze the composition of the cholesterol derivatives in the different subcellular fractions of StarD1 overexpressing hepatocytes. We first performed HPLC analysis of [¹⁴C]cholesterol derivatives in the methanol/water phases from the nuclear and non-nuclear fractions. FIG. 5 showed the 195 nm profiles, which nonspecifically detected double bonds containing molecules, and the specific radioactivity incorporated in each fraction derived from the [¹⁴C]cholesterol that had been added to the cultures after infection. The 195 nm profiles were relatively similar in the nuclear fraction (Fraction D) from cells infected either with the StarD1, CYP7A 1, or null recombinant viruses, except for an extra peak with retention time of 11.51 min, which was located between glycocholic acid (10.75 min) and taurochenodeoxycholic acid (12.50 min) (FIG. 5A). This 11.50 min peak was the only fraction that contained ¹⁴C-labelled cholesterol derivatives and was detectable only in StarD1 overexpressing cells (FIG. 5B). The non-nuclear fraction (Fraction A) from the StarD1 overexpressing cells contained two major [¹⁴C]cholesterol derivative peaks with retention times at 5 and 11.50 min. The 11.50 min peak was not detected in cells infected with either the control or the CYP7A1 viruses (FIG. 5C).

Further analysis of the cholesterol derivatives in the methanol/water phases extracted from mitochondrial, nuclear fractions, and culture media following overexpression of StarD1 protein showed that two [¹⁴C]cholesterol derivatives with retention times at 5.00 min and 11.5 min were found in mitochondria (FIGS. 6A and D). Furthermore, the 11.5 min peak was present only in mitochondria and nuclei but not in the media (FIGS. 6B and E) suggesting that a cholesterol-derived molecule should be generated in mitochondria and translocated to the nucleus. The peak with retention time at 5.00 min was found only in mitochondria and culture media, but not in the nucleus (FIGS. 6C and F) suggesting that the molecule presented in that peak was secreted from the cells. Therefore, the cholesterol derivative with a retention time of 11.50 min in the HPLC system, which is generated in the mitochondria and translocates to the nucleus, is defined as a nuclear oxysterol, and the other as a secretory oxysterol.

To characterize the chemical structure of the nuclear oxysterol, we proceeded to purify it by HPLC using C18 and silica columns, and analyze it by enzymatic digestion, TLC, and HPLC analysis. First, to detect the potential presence of sulfate group(s) in the nuclear oxysterol molecule, the purified nuclear oxysterol was digested with sulfatase. The products were extracted with methanol/chloroform, separated by Folch partitioning, and characterized by HPLC and TLC analysis. Following sulfatase treatment, the nuclear oxysterol products were extracted into the chloroform phase from the methanol/water phase and show the same retention time as 25-hydroxycholesterol, but not 27-, no 24-hydroxycholesterol in our HPLC system (FIG. 7A-C), and the same relative mobility as 25-hydroxycholesterol on the TLC plate (FIG. 7D), suggesting that the nuclear oxysterol derivative is a sulfated 25-hydroxycholesterol.

To confirm the chemical structure of the nuclear oxysterol derivative, the purified nuclear sulfated oxysterol was further analyzed by LC/MS/MS Mass Spectrometry. After we performed LC/MS (Q1 full scan mode, 350 to 550 atomic mass units, amu) under negative ionization mode, a prominent peak was observed at a retention time of 10.75 min in a selected ion chromatogram of mass ion at m/z=481 as shown in FIG. 8A. The Q1 full scan spectrum showed the peak at 10.75 min mainly contained molecule ions, m/z 480.1 and 481.5 (FIG. 8B). Further analysis showed the molecule ion, m/z 480.1 did not contain a sulfate group on hydroxyl group (m/z 97) (data not shown). However, the molecule ion, m/z 481.5, corresponds to sulfate group 97 and cholesterol (MW 386). This molecule ion was further analyzed by LC/MS/MS (product scan of m/z 481) under negative ionization mode (FIG. 8C). The characteristic fragment ions were observed at m/z=80(a), 97(b), 107, 123(c), 288, 465, and 59(d) in the product scan spectrum of m/z 481 (FIG. 8C). These observed fragment ions indicate that the nuclear oxysterol is a sulfated oxysterol with a sulfate group on 3-OH position (20) and a hydroxyl group on side chain, m/z 59, (molecular mass 482=80 (sulfate)+16 (O)+386 (cholesterol). Combined with data from HPLC, enzymatic digestion, and TLC analysis, the nuclear oxysterol derivative can be characterized as 5-cholesten-3β, 25-diol 3-sulphate (25-hydroxycholesterol 3-sulfate) as shown in (FIG. 8C).

Nuclear Oxysterol Derivatives from Primary Rat Hepatocytes Overexpressing StarD1 Increase Cholesterol Uptake and Bile Acid Synthesis.

To examine the function of the newly characterized nuclear oxysterol derivative in cholesterol homeostasis, the effects of the purified oxysterol on cholesterol uptake and bile acid synthesis were determined by measurement of cholesterol and conversion of [¹⁴C]cholesterol into methanol/water-extractable [¹⁴C]products in primary rat hepatocytes. The rate of cholesterol uptake was slightly higher in cells treated with nuclear extracts extracted from StarD1 overexpressing cells than those in hepatocytes treated with nuclear extracts isolated from CYP7A1 overexpressing cells or from control cells (data not shown). Similarly, rates of bile acid synthesis were significantly higher in hepatocytes treated with nuclear extracts from StarD1 overexpressing cells, and increased by 4-fold at 24 hrs after addition of the nuclear extracts. In contrast, nuclear extracts from CYP7A1 overexpressing cells did not significantly change bile acid synthetic rates compared with control nuclear extracts (FIG. 9A). To further confirm the role of the nuclear oxysterol derivatives in bile acid biosynthesis, the purified oxysterol derivative was dissolved in nuclear extracts (methanol/water phase) from cells infected with the control virus and added to primary rat hepatocytes. The results were very similar to those in cells directly treated with the nuclear extracts from the StarD1 overexpressing cells and in a time- and concentration-dependent manner (FIG. 9B), providing evidence that the nuclear oxysterol is a potent regulator of bile acid synthesis.

Discussion

Evidence for the first time to identify a novel nuclear regulatory oxysterol derivative, which is generated in mitochondria, translocated to the nucleus, and upregulates bile acid synthesis, gives rise to a new hypothesis that a new regulatory transduction pathway may play an important role in maintenance of intracellular cholesterol homeostasis.

The present results suggested that mitochondrial cholesterol transport proteins, such as StarD1, could serve as sensors of intracellular cholesterol levels. When cholesterol levels increase, StarD1 proteins deliver cholesterol into mitochondria where it is metabolized to 25-OH cholesterol 3-sulfate (the nuclear sulfated oxysterol). The generated nuclear oxysterol derivative is then translocated into the nucleus probably by binding and activating the nuclear oxysterol receptor(s). Without being bound by theory, it appears plausible that the nuclear sulfated oxysterol-nuclear receptor(s) complex may enter into the nuclei and regulate gene expression involved in cholesterol metabolism. A possible mechanism is proposed in FIG. 11.

25-Hydroxycholesterol 3-sulfate may be a Potential Authentic Ligand of Nuclear Sterol Receptor(s).

In the presence of StarD1 protein, cholesterol enters the mitochondria where it is oxidized to 25-hydroxycholesterol and is then sulfated to the nuclear sulfated oxysterol. The present results showed that both 25-hydroxycholesterol and 25-hydroxycholesterol 3-sulfate entered the nucleus. To date, 25-hydroxycholesterol has been believed to be the most potent regulator of gene transcription (21-23). To identify which one exhibits a more potent regulatory function, 25-hydroxycholesterol and 25-hydroxycholesterol 3-sulfate in nuclear extracts were separated by Folch partitioning: 25-hydroxycholesterol partitions to the chloroform phase and 25-hydroxycholesterol 3-sulfate partitions to the methanol/water phase. Interestingly, addition of the chloroform phase extracts containing 25-hydroxycholesterol to cells slightly increased the rates of bile acid synthesis (data not shown). However, the addition of methanol phase extracts dramatically increased bile acid synthesis (FIG. 9). Moreover, 25-hydroxycholesterol 3-sulfate is a water-soluble compound. It is thus reasonable to propose that 25-hydroxycholesterol 3-sulfate serves as a potent nuclear sterol regulators in vivo.

25-Hydroxylases (25-OHLase) and Hydroxycholesterol Sulfotransferase 2 (HS12) may be Involved in the Generation of the Potent Nuclear Sulfated Oxysterol.

StarD1 delivers cholesterol into the mitochondria and generates a novel nuclear sulfated oxysterol, 25-hydroxycholesterol 3-sulphate. This data suggests that a new pathway of cholesterol metabolism is responsible for generating this nuclear oxysterol. Our present report shows the presence of 25-hydroxycholesterol 3-sulfate in both the mitochondria and the nucleus but not in culture media, suggesting that 25-hydroxycholesterol 3-sulfate is generated in mitochondria and translocates exclusively to the nucleus. To biosynthesize this potent nuclear sterol regulator, two reactions should be involved: 25-hydroxylation and 3β-sulfation of cholesterol. It is not clear at this time whether 25-hydroxylation is catalyzed by CYP27A1 or 25-hydroxylase because 25-hydroxylase has not yet been identified in hepatocytes or in mitochondria, although it has been cloned (19). Since we did not see any cholesterol-3β-sulfate in the HPLC elution profile, we believe the first step is likely to be 25-hydroxylation of cholesterol catalyzed by 25-hydroxylase to form 25-hydroxycholesterol. Subsequently, sulfation of 25-hydroxycholesterol at the 3β position, is catalyzed by hydroxycholesterol sulfotransferases HST2(a, b), enzymes which have recently been cloned and identified (24;25). In addition, 25-hydroxycholesterol 3-sulfate may be glucuronidated for further catabolism and secretion via the bile as 24-hydroxycholesterol 3-sulfate (26). However, at the present time it is not clear that the [¹⁴C]-cholesterol derivative with a retention time of 5 min is the glucuronidation product of 25-hydroxycholesterol 3-sulfate (FIGS. 5 and 6).

Steroid sulfate conjugates may play an important role in the maintenance of cholesterol homeostasis. An interesting development in recent years has been the realization that steroid sulfoconjugates play important roles in well-characterized biological effects, such as serving as potent neuroexcitatory agents, which are distinct from the well-known role of unconjugated steroids as ligands for nuclear receptors to regulate gene expression (27). Several sulfated sterols have been reported to be widely distributed in steroidogenesis tissues (26) and to circulate in plasma at concentrations ranging from 328-924 μg/100 ml, with a blood production rate of 35-163 mg/day (28). A similar sulfated oxysterol derivative, 24-hydroxycholesterol 3-sulfate 24-glucuronide (based on MS/MS analysis) was reported to be in the serum and urine of children with severe cholestatic liver diseases (26). The 3-sulfate of 24-hydroxycholesterol is the major hydroxycholesterol sulfate found in meconium and infant feces (29;30), and is most likely excreted via bile. Bile excretion of this oxysterol would be impaired in cholestasis, leading to its increased concentration of 24-hydroxycholesterol 3-sulfate in hepatocytes and conceivably leading to its glucuronidation. However, where the compound came from and what function this oxysterol plays in the cholestatic liver is still a mystery. Although high levels of the double conjugate of 24-hydroxycholesterol are an indicator of grave liver disease, and can be used as a criterion for recommending liver transplantation, the physiological role and metabolism of this compound have never been identified. Sulfated sterols have been implicated in a wide variety of biological processes, e.g. regulation of cholesterol synthesis, sperm capacitation, thrombin and plasmin activities, and activation of protein kinase C isozymes (24). Furthermore, sulfated sterols can serve as a substrate for adrenal and ovarian steroidogenesis (31;32). Sulfated sterols play an important, but unclear, role in the normal development and physiology of skin, where an epidermal sterol sulfate cycle has been described (24). The present results show that the nuclear extract containing the sulfated oxysterol and the purified nuclear sulfated oxysterol dramatically increased the rates of bile acid synthesis, strongly suggesting that the nuclear sulfated oxysterol may play an important role in cholesterol metabolism.

Activation of LXRs by oxysterols is believed to be responsible for regulation of the metabolism of several important lipids, including cholesterol and bile acids (33). The identification of an LXR response element in the promoter of the rat cholesterol CYP7A1 suggested that LXRs play an important role in the regulation of cholesterol homeostasis(34;35). LXRα-deficient mice (LXRα−/−) dysregulate the CYP7A1 gene and several other important lipid-associated genes (3). Studies utilizing these animals confirmed the essential function of LXRα as a major sensor of dietary cholesterol and an activator of the bile acid synthetic pathway in mice. The finding of authentic oxysterol ligands for LXRs is one of the most important investigative methods for developing new therapeutic methods for the prevention and treatment of hyperlipidemia and atherosclerosis. To date, there is still only indirect evidence of the important role of oxysterols, such as 24-, 25-, or 27-hydroxycholesterol, as authentic ligands in the normal regulation of cholesterol homeostasis. Soluble and nuclear oxysterol-binding proteins with a high affinity for oxysterols exist, but the physiological ligands for these proteins have not yet been defined with certainty (36). The present report, with evidence showing that a potent regulatory nuclear sulfated oxysterol that is generated in the mitochondria translocates into the nucleus, provides a new clue regarding the role of oxysterol(s) in the regulation of intracellular cholesterol homeostasis. It is reasonable to hypothesize that the regulatory nuclear sulfated oxysterol generated in mitochondria translocates into nucleus, activates nuclear oxysterol receptor(s), and up-regulates bile acid synthesis. The nuclear sulfated oxysterol serves as a ligand of nuclear sterol receptor(s).

REFERENCES FOR EXAMPLE 1

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Example 2 Demonstration of Up-Regulation of LXR Targeting Gene Expression

Preliminary experiments have shown that the purified nuclear oxysterol up-regulates bile acid synthesis. To further investigate the mechanism of this activity, the effect of purified 5-cholesten-3β, 25-diol 3-sulphate on gene expression of LXR-targeted cholesterol transport proteins ABCA1, ABCG1, ABCG5, ABCG8, and LDLR was investigated. Purified nuclear sulfated oxysterol was added to primary hepatocytes in culture, and mRNA levels of the transport proteins were quantitated by real time RT-PCR. The primer sets and TagMan probes for detection of mRNA levels were purchased from AB Applied Biosystem (Foster City, Calif.) and the reactions were performed on an MJ Research DNA Engine Opticon instrument. The results are presented in FIG. 12 and show that the addition of purified nuclear oxysterol to primary hepatocytes in culture increased expression of ABCA1 (2-fold) and ABCG5 (1.6-fold), and ABCG8 (3 fold). The expression of ABCG1 is typically low in primary hepatocytes and did not change compared with control cells. The correlation of increased levels of the sulfated oxysterol in nuclei with increased levels of LXR targeting gene expression suggests that the nuclear oxysterol receptor, LXR, is activated by exposure to the sulfated oxysterol. These data do not rule out the activation of other nuclear receptor(s) as well.

Example 3 Synthesis of 5-cholesten-3β, 25-diol 3-sulphate

FIG. 12A shows a schematic illustration of the synthesis of the novel sulfated oxysterol of the invention by addition of a sulfate group to the 3β-position of 25-hydroxycholesterol. Synthesis was carried out as follows: A mixture of 25-hydroxycholesterol (0.1 mmol) and sulfur trioxide triethyl amine complex (0.12 mmol) in dry toluene was heated to 60 degree for 24 hours under nitrogen, then cooled and the solvent was evaporated under reduced pressure. The residue was purified by flash chromatography to afford the product as a white solid using the method of described above. FIG. 12B shows the mass spectrophotometric analysis of the HPLC purified product, which suggests that the sulfate group has been successfully added to 25-hydroxycholesterol.

FIGS. 12C and 12D show NMR data for the starting material, 25-hydroxycholesterol, and product, respectively. As can be seen, the resonance of C3 in the molecule has been shifted from 3.35 ppm in the original compound to 4.12 ppm in the product, suggesting that the desired product, 3β-sulfated 25-hydroxycholesterol, has been formed.

Example 4 A Nuclear Oxysterol, 25HC3S, Decreases Cholesterol Synthesis Via Inhibition of SREBP-1 Activation in Hepatocytes

Abstract

Recently, a novel oxysterol, 5-cholesten-3β, 25-diol 3-sulfate (25HC3S) was discovered, characterized, and identified in mitochondria and nuclei of primary hepatocytes following overexpression of the cholesterol transport protein, StarD1. This oxysterol was also detected in human liver nuclei. In the present study, 25HC3S was chemically synthesized. Addition of varying concentrations of 25HC3S to HepG2 cells markedly inhibited cholesterol biosynthesis and significantly decreased microsomal cholesterol. Real time RT-PCR and Western blot analysis shows that 25HC3S strongly decreased HMG CoA reductase mRNA levels in HepG2 cells and primary human hepatocytes. In comparison, 25-hydroxycholesterol (25HC) inhibited HMG CoA reductase mRNA levels in HepG2 cells but not in primary human hepatocytes when the cells were culture in serum-free media. Coincidentally, 25HC3S inhibited the activation of steroid response element binding protein (SREBP-1) in absence or presence of mevinolin and mevalonate, indicating that cholesterol biosynthesis inhibition occurred through blocking SREBP-1 activation, and subsequently the expression of HMG CoA reductase in the human hepatocytes. In conclusion, the presented findings indicate that 25HC3S regulates intracellular cholesterol biosynthesis via inhibiting SREBPs activation in human hepatocytes.

Introduction

The “acidic” pathway of bile acid biosynthesis is initiated by the mitochondrial enzyme sterol 27-hydroxylase (CYP27A1). Oxysterol intermediates of the “acidic” pathway such as 27-hydroxycholesterol (27HC) and 25-hydroxycholesterol (25HC) have been shown to be regulators of cholesterol homeostasis (1;2). These oxysterols represent regulatory molecules for the expression of many other genes encoding enzymes involved in cholesterol biosynthesis and transport (3-5). In theory, increased CYP27A1 activity in peripheral tissues could both down-regulate cholesterol synthesis through generating regulatory oxysterols and the steroid response element binding proteins (SREBPs) pathway, and enhance the cellular efflux of cholesterol, i.e. its elimination, via liver oxysterol receptor, LXR (6). However, the relationship between the CYP27A1 activity and intracellular cholesterol metabolism is unknown.

Previous reports showed that overexpression of the steroidogenic acute regulatory protein (StarD1), a protein which facilitates cholesterol transport into mitochondria, dramatically increases cholesterol catabolism to bile acids both in primary hepatocytes in culture and in vivo (7;8). These findings suggest that cholesterol delivery to the mitochondria, where the enzyme CYP27A1 is localized, is the rate-determining step for bile acid synthesis via the “acidic” pathway. Subsequently, StarD1 was detected in hepatocytes (9). Overexpression of StarD1 in vivo not only increases bile acid synthesis to the same level as overexpression of CYP7A1, but also produces a similar composition of bile acids in bile (8). As described in Examples 1-3, a novel oxysterol, 5-cholesten-3β, 25-diol 3-sulfate (25HC3S) was found and characterized in mitochondria and nuclei of primary hepatocytes following overexpression of StarD1. This oxysterol was also present in human liver nuclei (10). The results suggested that the oxysterol is synthesized in the mitochondria and translocated to the nuclei. Oxysteorls in nuclei should be able to play important roles in maintenance of intracellular cholesterol homeostasis. However, the function of this nuclear oxysterol, 25HC3S, remains unknown.

In the present study, we chemically synthesized 25HC3S and presented evidence that this oxysterol strongly regulates HMG CoA reductase, a key enzyme in cholesterol biosynthesis via SREBP regulatory system in hepatocytes.

Materials and Methods

Materials:

Cell culture reagents and supplies were purchased from GIBCO BRL (Grand Island, N.Y.); [¹⁴C]Cholesterol and [³H]25-hydroxycholesterol ([³H]25HC) from New England Nuclear (Boston, Mass.). [¹⁴C]27-OH Cholesterol was prepared as previously described (11). HepG2 cells were obtained from American Type Culture Collection (Rockville, Md.). Fetal bovine serum was obtained from Bio Whittaker (Walkersville, Md.). Tissue culture flasks were purchased from Costar Corp. (Cambridge, Mass.). The reagents for real time RT-PCR were from AB Applied Biosystem (Warrington WA1 4 SR, UK). The chemicals used in this research were obtained from Sigma Chemical Co. (St. Louis, Mo.) or Bio-Rad laboratories (Hercules, Calif.) unless otherwise specified. All solvents were obtained from Fisher (Fair Lawn, N.J.) unless otherwise indicated. The enhanced chemilluminescence (ECL) reagents were purchased from Amersham Biosciences (Piscataway, N.J.). The testosterone and 27HC were obtained from Research Plus Inc. (Bayonne, N.J.). LK6 20×20 cm thin layer chromatography (TLC) plates were purchased from Whatman Inc. (Clifton, N.J.). Nylon membranes were purchased from Micron Separation Inc. (Westborough, Mass.).

Chemical Synthesis of 5-cholesten-3β, 25-diol 3-sulfate

A mixture of 25HC (402 mg, 1 mmol) and triethylamine-sulfur trioxide pyridine complex (160 mg, 1 mmol) in 5 ml of chloroform was stirred at 25 degree for 7 days. After the solvent was evaporated at reduced pressure, the products were purified by HPLC using silica gel column and solvent of methylene chloride and methanol (5%) as mobile phase to afford the product as a white solid powder. The structure of the product was characterized by mass spectrum (MS) and nuclear magnetic resonance (NMR) spectroscopy analysis.

Mass Spectral Analysis

The synthesized compound was analyzed by a MDS Sciex ABI 4000 Triple Quadrapole Mass Spectrometer (MDS Sciex, Toronto, Canada) with a Turbo IonSpray ionization (ESI) source and the mass spectrometer was operated in negative ion modes and data were acquired using full scan mode as previously described (10).

Proton Nuclear Magnetic Resonance Spectroscopy

Samples were prepared for ¹H NMR analysis as described previously (12). Briefly, each sample, 1 mg, was dissolved in 0.5 ml of D₂O and lyophilized to remove exchangeable protons. The residue was dissolved in 0.5 ml of dimethyl sulfoxide-d6/D₂O (98:2, v/v). NMR spectra were obtained on spectrometers operating at ¹H frequencies of 300 MHz.

Cell Culture

HepG2 cells were grown in MEM containing non-essential amino acids, 0.03M NaHCO₃, 10% FBS, 1 mM L-glutamine, 1 mM sodium pyruvate and 1% Pen/Strep and incubated at 37° C. in 5% CO₂. When cells reached at ˜90% confluency, the oxysterols in DMSO or in ethanol (final concentration, 0.1%) and/or [1-¹⁴C]acetate for cholesterol synthesis was added or otherwise as indicated. Microsomal and cytosol fractions were isolated from broken cells as described previously (10).

Primary human hepatocytes were purchased from an NIH-approved facility (Liver Tissue Procurement Distribution System, Univ. of Minnesota). Cells were obtained from a random sampling of males and females 18-69 yr of age. Experiments were performed as cells became available to corroborate findings in experiments conducted in HepG2 cells as previous described (13).

Determination of Cholesterol Biosynthesis by TLC and HPLC

After incubation of HepG2 cells with media containing 25HC3S for 6 hrs, cultures in 25-cm2 flasks were given 3 ml of the same fresh medium containing 5 μCi of the [1-¹⁴C]acetate. After 4 hr incubation at 37° C., the media were removed and the cells were washed twice with phosphate-saline buffer (PBS), harvested with rubber police as described, and collected in Eppendoff tubes. The cells were sedimented by centrifugation and the pellets were washed three times by resuspension and sedimentation. The pellets were resuspended in 0.3 ml of PBS. The total lipids were extracted and separated by adding 3 volume of chloroform:methanol (v/v, 1:1). [¹⁴C]Cholesterol and 27-hydroxycholesterol was isolated into chloroform phase and separated on TLC (tuluene:acetyl acetate, 2/3, v/v/). [1-¹⁴C]acetate derivatives were visualized by Image Reader, Fujifilm BAS-1800 II.

HPLC Analysis of Cholesterol Derivatives

[1-¹⁴C]Acetate derivatives in the chloroform phase were analyzed by HPLC on an Ultrasphere Silica column (5μ×4.6 mm×25 cm; Backman, USA) using HP Series 1100 solvent delivery system (Hewlett Packard) at 1.3 ml/min flow rate. The column was equilibrated and run in a solvent system of hexane:isopropanol:glacial acetic acid (965:25:10, v/v/v), as the mobile phase. The effluents were collected every 0.5 min (0.65 ml per fraction) except as indicated. The counts in [¹⁴C]acetate derivatives were determined by Scintillation Counting. The column was calibrated with [¹⁴C]cholesterol, [³H]25HC, and [¹⁴C]27-hydroxycholesterol.

Determination of HMG CoA Reductase mRNA by Real-Time RT-PCR

Total RNA was isolated from HepG2 cells using SV Total RNA Isolation Kit (Promega). Two μg of total RNA was used for first-strand cDNA synthesis as recommended by manufacture (Invitrogen). Real-time PCR was performed using SYBR Green on ABI 7500 Fast Real-Time PCR System (Applied Biosystems). The final reaction mixture contained 5 ng of cDNA, 100 nM of each primer, 10 μl of 2×SYBR® Green PCR Master Mix (Applied Biosystems), and RNase-free water to complete the reaction mixture volume to 20 μl. All reactions were performed in triplicate. The PCR was carried out for 40 cycles at 95° C. for 15 s and 60° C. for 1 min. The fluorescence was read during the reaction, allowing a continuous monitoring of the amount of PCR product. The data was normalized to internal control-β-actin or GAPDH mRNA. The sequences of primers for HMG CoA reductase used in real-time PCR are ACCTTTCCAGAGCAAGCACATT (SEQ ID NO: 1) (Forward) and AGGACCTAAAATTGCCATTCCA (SEQ ID NO: 2) (Reverse).

Western Blot Analysis

Microsomal proteins (80 μg) were separated on a 7.5% SDS-polyacrylamide denaturing gel according to the method of Laemmli (14). Following SDS-PAGE (Hoefer Vertical Slab Gel Unit; Heofer, San Francisco, Calif.), proteins were electrophoretically transferred overnight (4° C.) to Immobilon-P membranes using a Hoefer Trans Blot Electrophoretic Transfer cell. The membranes were then blocked for 90 minutes (25° C.) in blocking buffer (PBS, pH 7.4, 0.1% Tween, 5% non-fat dry milk). Proteins were then incubated for 90 minutes (25° C.) or overnight (4° C.) with a rabbit polyclonal IgG (1:2500-1:20,000) against human SREBP-1, SREBP-2, or HMG CoA reductase. After washing, a secondary antibody (goat anti-rabbit IgG-Horse-radish peroxidase conjugate, 1:2500) was added to the blocking solution (25° C., 90 minutes). Protein bands were detected using the Amersham ECL plus Kit.

Results

Chemical synthesis of the nuclear oxysterol, 5-cholesten-3β, 25-diol 3-Sulfate To study the role that 25HC3S may play in cellular cholesterol homeostasis, this oxysterol was chemically synthesized. Using a modified triethylamine-sulfur trioxide complex protocol as described in Methods, the nuclear oxysterol was successfully synthesized as outlined in FIG. 12A. MS analysis of the synthesized compound shows the same molecular mass ion, m/z 481 as the “authentic” nuclear oxysterol (FIG. 12B). ¹H NMR analysis shows that the proton resonance at C3 with multiple small (1.5 Hz) splits near 3.65 ppm in the spectrum of 25-OH cholesterol (starting material) (FIG. 12C) is shifted to 4.20 ppm in the product spectrum (FIG. 12D) as indicated, which confirms that HSO₃ ⁻ group is added at the C3 position of 25-OH cholesterol. HPLC analysis shows the same retention time of the synthesized oxysterol as the authentic nuclear oxysterol (data not shown). Combined with the results from the MS analysis, it is concluded that the synthesized molecule is 5-cholesten-3β, 25-diol 3-sulphate (25HC3S). The results also confirmed the structure of the nuclear oxysterol as previously reported (10 and Examples 1-3).

25HC3S Inhibits Cholesterol Biosynthesis and Decreases Cholesterol Levels in Microsomal Fractions

To examine the effects of 25HC3S on cholesterol biosynthesis, the rates of cholesterol synthesis were determined by adding [1-¹⁴C]acetate following the addition of varying concentration of 25HC3S to HepG2 cells in culture. FIG. 13 summarizes the effects of 25HC3S on cholesterol biosynthesis. After incubation of the cells in the media containing 25HC3S for 6 hrs and [¹⁴C]acetate for additional 4 hrs, total lipids were extracted and partitioned. Neutral lipids in the chloroform phases including [¹⁴C]acetate derivatives were analyzed by TLC and HPLC, TLC analysis shows that [¹⁴C]cholesterol ether, [¹⁴C]cholesterol, and [¹⁴C]25HC were synthesized but no detectable [¹⁴C]27HC (FIG. 13A). Free [¹⁴C]cholesterol was found to be significantly decreased following addition of 25HC3S while the other labeled sterols did not significantly change. The decreases were concentration dependent (FIG. 13A). The decreased amounts of [¹⁴C]cholesterol bands on the TLC were confirmed by HPLC analysis as shown in FIG. 13B. The results from three experiments are summarized in FIG. 13C.

To study the distribution of intracellular cholesterol following the addition of 25HC3S, microsomal, nuclear, and cytosol fractions were isolated and total lipids from each fraction were extracted with chloroform/methanol/water. The cholesterol concentration in each fraction was determined by HPLC after cholesterol oxidase treatment which converts sterols to 3-oxo-β4 derivatives. As shown in FIG. 14, 25HC3S significantly decreased cholesterol levels in the microsomal fraction (left panels) as well as 25HC (right panels) but not in cytosol and nuclear fractions (data not shown). The decreases in cholesterol concentration were dose dependent as indicated in FIG. 14. It was also observed that no 25HC could be detected in the fractions from cells treated with 25HC3S. In contrast, dose dependent levels of 25-hydroxycholesterol could be detected in the fractions treated with 25HC as shown in FIG. 14 (right panels). These results suggest that 25HC3S directly inhibits cholesterol biosynthesis and decreases cholesterol levels in microsomal fractions and not through its degradation to be 25HC.

The Nuclear Oxysterol Inhibits Cholesterol Biosynthesis by Decreasing HMG CoA Reductase mRNA Levels

To investigate how 25HC3S inhibits cholesterol biosynthesis, total mRNA were isolated from HepG2 cells following incubation in 10% FBS fresh media containing different concentrations of 25HC3S (FIG. 15A). The mRNA levels of HMG CoA reductase were determined by real time RT-PCR. As shown in FIG. 15A, there was concentration dependent decreases in HMG CoA reductase mRNA following the addition of 25HC3S to the cells in culture. The addition of 25HC3S to HepG2 cells also lead to a marked decrease in the levels of HMG CoA reductase protein (FIGS. 15C and D). Western blot analysis shows that 50% of its protein level was decreased in a concentration dependent following the addition of 25HC3S to culture media (FIGS. 15C and D).

To compare with 25HC, mRNA levels of HMG CoA reductase in 25HC or 25HC3S-treated HepG2 cells were analyzed by real time RT-PCR. Both of the compounds can inhibit HMG CoA reductase in a similar fashion as shown in FIG. 16A. To further distinguish whether 25HC3S inhibits HMG CoA reductase expression via lipids uptake in the culture media, HepG2 cells were incubated in media containing lipid-depleted serum for 2 hrs, which increases expression of HMG CoA reductase by four-fold as previously reported (15). Cells were incubated for another 6 hrs following the addition of 25HC3S. Real time RT-PCR analysis shows that 25HC3S still strongly inhibits HMG CoA reductase mRNA levels (FIG. 16B). Interestingly, 25HC shows much more potent inhibition of the reductase mRNA in lipid-depleted media as compared to cells cultured in medium containing FBS (FIGS. 16A and 5B). Under these conditions, 25HC is more potent inhibition than 25HC3S on HMG CoA reductase mRNA levels. The result of inhibition of HMG CoA reductase by 25HC is consistent with previous report (16;17). To confirm its physiological role that 25HC3S plays in the cholesterol biosynthesis, primary human hepatocytes (PHH) cultured in serum-free media were used. Surprisingly, 25HC did not significantly affect the levels of HMG CoA reductase mRNA (˜15% at 25 mM). In contract, 25HC3S inhibited HMG CoA reductase mRNA at a similar level (˜75%) as that in HepG2 cells (FIG. 16C). HPLC analysis showed that the increasing levels of 25HC in the cells are concentration dependent (FIG. 17A-E), indicating that 25HC can still enter to the cells under this culture condition as in FBS containing media. These results suggest that 25HC3S and 25HC inhibit HMG CoA reductase mRNA by different mechanisms.

The Nuclear Oxysterol Inhibits HMG CoA Reductase Expression by Inhibition of Both SREBP-1 and SREBP-2 Activation

It has been well documented that HMG CoA reductase gene expression is regulated by SREBP-1 and SREBP-2 (18). When SREBPs are activated, the cholesterol biosynthesis will increase (19). To study whether SREBP regulatory system is involved in the inhibition of HMG CoA reductase mRNA levels and inhibition of cholesterol biosynthesis by 25HC3S, total cellular protein was extracted from HepG2 cells treated with 25HC3S or 25HC at different concentration (FIG. 18). The precursor and mature forms of SREBPs were determined by Western blot analysis. As expected, the decreases of the mature forms of SREBP-1 and the increases of the precursor form of SREBP-1 following addition of 25HC3S and 25HC were dose dependent (FIGS. 18A and 7B). However, the mature form of SREBP-2 only slightly decreased (FIGS. 18A and 7B). It was observed that the activation of SREBP-1 was much more sensitive to the treatment with 25HC and 25HC3S than that of SREBP-2. The inhibition of SREBP-1 maturation fits the decrease in HMG CoA reductase mRNA, at 3 μM of 25HC or 12 μM of 25HC3S, 85% of the mRNA of HMG CoA reductase and SREBP1 was inhibited. To confirm the mechanism, HepG2 cells were incubated in media containing 50 μM of mevinolin and 0.5 μM of mevalonate. Under this condition, SREBPs and HMG CoA expressions are upregulated. Following the treatment with 25HC or 25HC3S, SREBPs activation in the cells was determined by Western blot as described above. As shown in FIGS. 18C and 8D, 25HC3S shows more potent inhibition on SREBP-1 activation than 25HC. Thus, the nuclear oxysterol, 25HC3S, is most likely to inhibit the activation of SREBP-1 and subsequently inhibits the expression HMG CoA reductase.

Discussion

The current study shows that the chemically synthesized oxysterol, 25HC3S as well as 25HC, inhibits the cholesterol biosynthesis and the expression of the key enzyme, HMG CoA reductase. Unlike 25HC, 25HC3S inhibits this expression in primary human hepatocytes as well as HepG2. 25HC3S was found in the human liver nuclei and its levels were dramatically increased following overexpression of mitochondrial cholesterol delivery protein, StarD1, in primary rat hepatocytes, indicating that 25HC3S is most likely synthesized in the mitochondria and translocated to the nuclei for the regulation of gene expression involved in cholesterol homeostasis (10). The present results provided evidence that the oxysterol, 25HC3S, plays an important role in maintenance of the intracellular cholesterol homeostasis, and suggested that the StarD1 protein may serve as a sensor of intracellular cholesterol levels. When cholesterol levels are too high in the cells, StarD1 protein may deliver cholesterol to mitochondria where cholesterol is converted to be potent regulatory oxysterols such as 25HC, 27HC, and 25HC3S. Those oxysterols play important roles in maintenance of cholesterol homeostasis.

Theoretically, the added oxysterols can be metabolized after they enter into cells: 25HC can be sulfated by hydroxysteroid sulfotransferase 2B1b (SULT2B1b) to be sulfated 25HC or sulfated 25HC can be degraded by sulfatase to be 25HC. Thus, which one serves as a potent regulator in the maintenance of cholesterol homeostasis is questionable. Our HPLC analysis data shows that no 25HC was generated following addition of 25HC3S up to 50 μM (FIG. 17, left panels) suggesting that 25HC3S was not degraded during the culture time. In contrast, the peak of 25HC increased following the addition of the 25HC and the increasing is dose dependent (FIG. 17, right panels). Although the results can not rule out the possibility that a small portion of the oxysterol was converted to be the sulfated 25HC, there is no evidence that 25HC works as the sulfated form at present.

25HC3S and 25HC inhibit cholesterol synthesis by two different mechanisms, both involving the proteins that control sterol regulatory element-binding proteins (SREBPs), membrane-bound transcription factors that activate genes encoding enzymes of lipids biosynthesis. In sterol-depleted cells, SREBP cleavage-activating protein (SCAP) escorts its bound SREBP to the Golgi apparatus where the SREBP is processed sequentially by two membrane-embedded proteases. The NH₂-terminal domain released by the process can enter the nucleus where it activates transcription of the gene encoding HMG CoA reductase and more than 30 other genes whose products are necessary for lipid synthesis (18). When 25HC is delivered to cells in ethanol or when cholesterol is delivered in LDL, SCAP becomes trapped in the ER. The bound-SREBP is no longer carried to the Golgi apparatus, and the NH2-terminal domain can not enter the nucleus (20). As a result, transcription of the lipid biosynthetic genes declines. Retention of the SCAP-SREBP complex in the ER is mediated by the sterol-induced binding of SCAP to Insigs (Insig-1 and Insig-2) in the ER membrane (20;21). When mixtures of cholesterol and 25HC are added to cultured cells, SCAP is induced to bind to Insig 1 and Insig-2, and thus can not transport SREBPs to the Golgi body (3) and to be activated. J.L. Goldstein laboratory provided evidence that cholesterol interacts with SCAP directly by inducing it to bind to Insigs, whereas 25HC works indirectly through a putative 25HC sensor protein that elicits SCAP Insig binding (3). However, what the 25HC sensor protein is unknown.

The present study showed that 25HC strongly inhibits HMG CoA reductase in the presence of the lipids depleted serum both in HepG2 (FIG. 18) (˜90%), similar inhibits as 25HS3S does in the presence of serum (˜70%), but weakly inhibits in PHH when the cells were cultured in serum free media (FIG. 178) (˜15%). These results suggest that PHH does not express 25-hydroxycholesterol sensor protein(s) under this culture system. However, 25HC3S can inhibit the enzyme expression to similar levels either presence or absence of serum or lipids depleted serum in both hepG2 and PHH indicating that 25HC3S can directly regulate the gene expression. The major reason could be that 25HC3S is much more hydrophilic than 25-hydroxycholesterol and is water soluble, which makes the molecule freely self-transport after this molecule enters the cells.

The oxysterol, 25HC3S, inhibits the cholesterol biosynthesis through inhibiting the activation of SREBP-1 and SREBP-2, and subsequently inhibit the expression HMG CoA reductase in HepG2 cells and primary human hepatocytes. Based on the microarray data from transgenic and knockout mice, both SREBP-1 and SREBP-2 activation stimulate HMG CoA reductase by 30 folds and 38 folds, respectively (18). SREBP-1 also strongly stimulates fatty acid synthase but not SREBP-2 (18). However, it is not clear whether SREBP-1 and SREBP-2 are directly mediated by 25HC3S to regulate the expression regulation of the key enzyme, HMG CoA. Our results clearly show that 25HC3S as well as 25HC are much more potent in inhibiting SREBP-1 activation than SREBP-2. We believe that 25HC3S inhibits SREBP-1 activation and subsequently inhibits HMG CoA reductase resulting in the decline of cholesterol biosynthesis.

REFERENCES FOR EXAMPLE 4

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Example 5 In Vitro and In Vivo Testing of the Effect of the Nuclear Oxysterol 25HC3S on Triglyceride and Cholesterol Levels

The effect of the addition nuclear oxysterol on cholesterol levels was tested in human hepatocyte HepG2 cell line, and the results are presented in FIG. 19. As can be seen, administration of the nuclear oxysterol resulted in a significant decrease in the level of cholesterol detected in microsomal fractions of the HepG2 cells, when compared to the control (25HC3S concentration=0).

Female mice (20-25 g weight) were injected intravenously 3× with or without 72 μg of the nuclear oxysterol in 100 μl of 0.9% NaCl containing 3 μl DMSO. Injections were made every 12 hrs. 30 hours following the first injection, blood was harvested for analysis of triglycerides and cholesterol. The results are presented in FIG. 20, which shows that injection of the nuclear oxysterol decreased triglycerides in serum by 50% and decreased cholesterol levels by 28% (n=4). Further, injection of the nuclear oxysterol did not increase the activities of alkaline phosphotase, serum glutamate pyruvate transaminase (SGPT), and serum glutamic-oxaloacetic transaminase (SGOT) in sera suggesting that the nuclear oxysterol is non-toxic (data not shown).

To test the effect of diet on cholesterol levels, mice were provided with either a normal or high cholesterol diet for 8 days, and treated with the nuclear oxysterol as described above. Liver tissues were collected for pathohistochemistry studies (Sudan IV staining) and the results showed that administration of the oxysterol significantly decreased triglyceride levels in liver tissues of mice that were fed normally and also of mice that were fed a high cholesterol diet (data not shown).

This study demonstrates that this natural nuclear oxysterol, 25HC3S, could serve as a potent drug for the treatment and prevention of hypercholesterolemia and hyperlipidemia, and related diseases such as atherosclerosis.

While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. Accordingly, the present invention should not be limited to the embodiments as described above, but should further include all modifications and equivalents thereof within the spirit and scope of the description provided herein. 

1. Substantially purified 5-cholesten-3β, 25-diol 3-sulphate having the following formula:


2. A cholesterol and/or triglyceride-lowering composition, said composition comprising 5-cholesten-3β, 25-diol 3-sulphate, and a pharmaceutically acceptable carrier.
 3. A method for lowering serum cholesterol and/or triglyceride levels in a patient in need thereof, said method comprising the step of administering 5-cholesten-3β, 25-diol 3-sulphate to said patient in an amount sufficient to lower serum cholesterol levels in said patient.
 4. A method to treat or prevent a pathological condition associated with high serum cholesterol levels in a patient in need thereof, said method comprising the step of administering 5-cholesten-3β, 25-diol 3-sulphate to said patient in an amount sufficient to lower serum cholesterol and/or triglyceride levels in said patient and prevent or treat said pathological condition.
 5. The method of claim 4, wherein said pathological condition is selected from the group consisting of hypercholesterolemia, hyperlipidemia, and atherosclerosis.
 6. A method of increasing cholesterol secretion or degradation in a cell, comprising the step of increasing a level of 5-cholesten-3β, 25-diol 3-sulphate in said cell. 